This invention relates to stabilized alkali polysaccharides and methods for the preparation thereof.
Derivatives of polysaccharides, such as cellulose ethers, have a wide diversity of properties ranging from organic-soluble thermoplastic products useful in film applications to water-soluble food additives. Such derivatives, particularly the polysaccharide ethers, are generally manufactured from a preformed polymer, e.g., cellulose, that contains chains of .beta.-anhydroglucose rings. In the ether derivatives, these rings are substituted with various monovalent organic radicals such as methyl, ethyl, benzyl, hydroxyethyl, hydroxypropyl, carboxymethyl and combinations thereof. The chemical nature, quantity and distribution of the substituent groups of the polysaccharide govern such properties as solubility, surface activity, thermoplasticity, film characteristics and biodegradability.
In the commercial production of polysaccharide ethers and other polysaccharide derivatives, the polysaccharide is first mixed with strong alkali, e.g., aqueous sodium hydroxide, in the presence of air. Subsequently, this alkali polysaccharide is reacted with etherifying agent, e.g., alkyl halide, monochlorocarboxylic acid or vicinal epoxide. Usually the alkyl chloride contains from 1 to 2 carbon atoms, the chloro acid contains from 2 to about 4 carbon atoms and the epoxide can contain from 2 to 4 carbon atoms and it can be halogenated, e.g., as in epichlorohydrin. The alkyl chlorides react to form alkyl ethers, the chloro acids form carboxyalkyl ethers and the epoxides form hydroxyalkyl ethers. Mixtures of alkyl chloride and vicinal epoxide form derivatives having both alkyl ether and hydroxyalkyl ether groups on the polysaccharide. A mixture of alkyl chloride and chlorocarboxylic acid yields a polysaccharide having both alkyl ether and carboxyalkyl ether groups. If desired, all three types of etherifying agents can be mixed for reaction with the polysaccharide to form a polysaccharide derivative having alkyl, carboxyalkyl and hydroxyalkyl ether groups on the polysaccharide. This reaction can be effected in a dry medium wherein the alkali polysaccharide, after preparation, is reacted with nonaqueous liquid reactants such as the aforementioned etherifying agents in quantities insufficient to make a slurry. Alternatively, the reaction can be effected in the presence of small quantities of water, with or without the addition of an inert diluent.
Unfortunately, the preparation of the polysaccharide derivatives by the aforementioned conventional procedures often yields products having poor quality. This poor quality is believed to be caused by the lack of uniform distribution of the substituent groups on the polymer backbone of the polysaccharide derivative. For example, when such substituents are uniformly distributed on the polysaccharide backbone, the polysaccharide derivative will dissolve easily in solvent and form superior films. When such distribution is not uniform, the resulting polysaccharide derivative does not dissolve easily and has poor film forming properties when used as a coating. In addition, the etherifying agents such as alkyl halides not only react with the polysaccharide but also react with water which may be present in the reaction. Thus, in order to increase the effective utilization of the etherifying agent, highly concentrated aqueous solutions of alkali are employed. This practice prevents the uniform dispersion of alkali in the polysaccharide which causes nonuniform distribution of ether groups in the resulting polysaccharide derivative. Finally, the use of such concentrated alkali often causes partial degradation of the polysaccharide, thereby causing the formation of polysaccharide derivatives having molecular weights lower than desired.
Various techniques have been employed in the past to overcome this problem of nonuniform distribution of substituent groups on the polysaccharide. For example, attempts to modify polysaccharides in homogeneous solutions of dimethyl sulfoxide in the presence of formaldehyde as described by Semour and Johnson, Polymer Preprints 17, 382 (1976) or in dimethyl acetamide in the presence of lithium chloride as described by C. L. McCormick et al., ACS Symposium Series 121, 371 (1980) involve expensive solvents and do not lead to quantitative substitutions. Alternately, the polysaccharide may be dispersed in an inert solvent such as isopropanol or tert-butanol prior to treatment with concentrated alkali solutions to assure a more even dispersal of hydroxide ions in the polysaccharide matrix. However, all of these methods employ expensive procedures and do not satisfactorily eliminate the nonuniform distribution of substituent groups in the polysaccharide. Moreover, many do not adequately alleviate the problem of alkali degradation of the polysaccharide.
In view of the foregoing deficiencies of conventional techniques for preparing derivatives of polysaccharides, it is highly desirable to provide an improved method for preparing such derivatives wherein uniform distribution of substituents is achieved without significant degradation of the polysaccharide.